Some details... (I know Sumner already knows this - but Ill repeat it anyhow)
The advice for paralleling the two batteries regarded charging using Sumner's high amp generator (capable of something like 50 amps). In this case, the generator was able to put out much higher current than the single battery / regulator acceptance would allow to be input to the battery. I.e., even though the generator was able to produce about 50 amps, the acceptance of the single battery was limiting charging to something like 20 amps. But if the batteries were charged in parallel, the current from the generator would be maybe 40 amps, ie twice - and the generator could be run for a shorter amount of time. The charging in parallel is mostly important if your charge capability is in excess of a single battery acceptance and you want to minimize how long the charger/generator is running. If your charge capablity is much under the battery acceptance, charging in parellel wouldnt add any value.
Also, when you’re running something like a fridge, it’s a large daily power drain. I think if you have a fridge, running it off a larger battery would result in smaller cycle percentage which I think is easier on the battery. So... I think in the case of the fridge, also running the 12 volt batteries in parallel would make them last longer. I think Sumner also said he generally turned the fridge off at night which sounds like a good idea if your getting the power from solar - easier on the batteires.
In my case, I don’t have a fridge but wanted the extra power capacity and I think its fine for others to put in dual battery switch - but I did not want one of these - I think to some extent, adding the switch makes it more likely that you will need it for a backup battery. So I had my 12 volt batteries in parallel and these were kept charged all the time either with solar or a plugged in AC charger. When I would check the water in the batteries, one battery would always use way more water than the other (there is a theory why this happens).
So really, the only reason I went to two six volt batteries is to get rid of this problem where one battery would use a lot more water than the other. After two years with the golf cart batteries, this problem does seem to be gone.
